1. Technical Field
This invention relates to fuel processing systems, and more particularly fuel processing systems used to generate hydrogen from organic fuels for fuel cell use.
2. Background Information
Fuel cell systems, including the catalytic components of a fuel processing system for converting organic fuel to hydrogen, and especially the anode side of the fuel cell itself, generally require purging upon shut-down and/or start-up to remove residual hydrogen (upon shut-down) and to remove air upon start-up. This is necessitated for several reasons, including the elimination of the potential for flammable mixtures of hydrogen and oxygen; minimizing performance degradation of fuel processing system catalysts and electrode catalysts; and prevention of hazardous material formation during the start-up and shut-down process. The latter may occur, for example, in reformers containing nickel as catalyst. Nickel reacts with carbon monoxide forming a toxic nickel carbonyl. Common practice is to purge components with inert gas such as nitrogen or nitrogen mixed with other gases harmless to the component being purged. For example, U.S. Pat. No. 4,537,839 describes using inert gases (defined therein as gases xe2x80x9csubstantially free of hydrogenxe2x80x9d), such as product gases from a catalytic combustor, to purge a fuel cell. U.S. Pat. No. 5,248,567 also describes the use of a fuel cell purge gas from which the combustion elements (mainly oxygen and reactive carbon) have been removed.
In fuel cell power plants, it is also known to use inert gas, such as nitrogen, to purge (upon shut-down) components of fuel processing systems that convert organic fuels, such as gasoline or natural gas, to hydrogen. It is desirable to avoid the cost and complexity of providing an inert purge gas for either the fuel cells or the fuel processing system.
In accordance with the present invention, a fuel cell system that includes a fuel reformer for converting an organic fuel to hydrogen is shut-down by disconnecting the fuel cell from its load, halting the flow of organic fuel to the reformer, and purging the reformer of residual hydrogen by flowing air through the reformer.
If the reformer is a steam reformer or autothermal reformer, it may be purged simultaneously with steam and air; however, it is preferred to purge first with steam and then with air. There will be residual raw fuel upstream of the reformer immediately upon shut-down. As the purge occurs, the fuel will enter the still hot reformer and react. An initial steam purge is desirable because it helps to maintain the correct reactant ratios during the purge process and insures that no undesirable reactions, such as carbon formation, occur during this period. Steam also provides a buffer between a fuel rich environment and an air environment mitigating safety concerns.
If, as is often the case, the fuel processing system includes one or more other components, such as a shift converter, selective oxidizer, and/or desulfurizer, those components may also be purged of residual hydrogen using air, and preferably by passing air in series through the components, with the purge gas output of one component passing into and through the next component. If a component uses a catalyst that is not tolerant of oxygen, that component may be bypassed and purged by conventional means, such as with inert gas.
After passing through the reformer and/or other fuel processing components, the purge gases may be directed through the anode flow field of the fuel cell to purge residual hydrogen from the anode side of the cell. Alternatively, the purge gases may be vented from the fuel cell system, such as to atmosphere, without passing them through the fuel cell.
Rather than passing the purge air through the fuel processing components in series, components may be purged in parallel, each with its own individual flow of purge air that is vented to atmosphere.
In accordance with the present invention it is preferred to purge the fuel processing components with a volume of air at least three times the volume of the largest component being purged to assure sufficiently complete removal of the hydrogen, which would be to less than 4% by volume (the flammability limit of hydrogen in air) and preferably to less than 1%. In any event, the purge gas moves through the reactors more or less as a front. Reaction on the various catalyst beds reduces the hydrogen content at the leading edge of the front to essentially zero.
Although a blower may be used to push the purge air and other gases through the fuel processing components and associated plumbing, in one embodiment of the present invention a passive purge is contemplated. In that embodiment, the fuel processing components to be purged may be arranged in a vertical stack. One or more valved purge air inlets are located at low points of the fuel processing stack and one or more valved purge gas outlet are located at high points in the stack. During fuel cell operation both the purge air inlets and outlets are closed. Upon shut-down of the fuel cell, after turning off the flow of fresh fuel the purge air valves are opened. Air enters the inlet(s) and, by natural circulation, rises through the fuel processing stack, along with the residual hydrogen and any other gases. The gases leave the fuel processing stack through the outlet(s). Eventually only air remains within the components. If a vertical fuel processing stack is not desired or practical for an installation, the components may be individually and separately passively purged.
Although a fast air purge, using a blower or the like, is preferred, the passive air purge has the advantage of requiring little or no power. For, example, the purge air valves may be designed to open when de-energized. In that case, the purging is accomplished with no external power source.
If the fuel cell anode flow field is also to be purged with air (instead of an inert gas), speed is more critical, and the air front should move through the anode flow field in no more than about 1.0 seconds, and preferably less than 0.2 seconds. This would require a blower or the like.
The direction of the air purge through a component is not critical. In other words, the purge air may be directed through the components in either the same or the opposite direction from the flow of fuel during fuel processing. However, to maximize the speed of natural circulation in a vertical fuel processing stack (by taking advantage of gas density differences due to temperature and gas composition), it is preferred to stack the fuel processing components so as to enable purging in a direction opposite to the fuel processing flow direction.
The foregoing features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings.